Polyurethane/urea elastomers in the forms of fibers and films have found wide acceptance in the textile industry. The term “spandex”, often used to describe these elastomers, refers to long chain synthetic polymers made up of at least 85% by weight of segmented polyurethane. The term “elastane” is also used (e.g., in Europe) to describe these polymers. Spandex is used for many different purposes in the textile industry, especially in underwear, form-persuasive garments, bathing wear, and elastic garments or stockings. The elastomeric fibers may be supplied as core spun elastomer yarns spun round with filaments or staple fiber yarns or as a staple fiber in admixture with non-elastic fibers for the purpose of improving the wearing qualities of fabrics which are not in themselves highly elastic.
In the past, thread made of natural rubber was the only material available to provide elasticity to fabrics. Spandex, originally developed in the 1950s, has numerous advantages over such rubber filaments. The most important of these is its higher modulus. Typically, for a given denier, spandex has at least twice the recovery, or retractive power, of rubber. This enables stretch garments to be manufactured with less elastic fiber and thus be lighter in weight. Additional advantages over natural rubber include the ability to obtain spandex in much finer deniers, higher tensile strength and abrasion resistance, and in many cases, higher resilience. Additionally, spandex exhibits improved resistance to many cosmetic oils, to solvents (for example, those used in dry cleaning), and a high resistance to oxidation and ozone as well. Furthermore, in contrast to rubber filaments, spandex fibers can be dyed relatively easily with certain classes of dyestuffs.
Highly stretchable, low modulus fibers are particularly desirable. U.S. Pat. Nos. 4,772,677 and 5,000,899 disclose methods for preparing fibers of this type. The method disclosed in U.S. Pat. No. 5,000,899, however, requires raw materials which make the product fibers economically unattractive.
Preparation of polyurethane elastomers by the polyaddition process from high molecular weight, substantially linear polyhydroxyl compounds, polyisocyanates and chain lengthening agents which have reactive hydrogen atoms by reaction in a highly polar organic solvent is known. The formation of fibers, filaments, threads, and films from these solvent-borne polyurethane elastomers and by reactive spinning is also known. See, e.g., U.S. Pat. Nos. 3,483,167 and 3,384,623 which disclose preparation of spandex fibers from isocyanate-terminated prepolymers prepared with polymeric diols.
Spandex made with PTMEG-derived prepolymers and polymers does not have the elongation or the low hysteresis of natural rubber but it is characterized by improved retractive power, higher tensile strength and the ability to better withstand oxidative aging. These improved features have made PTMEG-derived spandex the industry standard, despite the difficulties associated with PTMEG-derived prepolymers and polymers, and the relatively high cost of PTMEG itself.
For the reasons discussed above, the commercially preferred polymeric diol is polytetramethylene ether glycol (PTMEG). PTMEG is a solid at room temperature and produces prepolymers, particularly, diphenylmethane diisocyanate (“MDI” ) prepolymers having extremely high viscosities.
However, despite the inherent difficulties of handling PTMEG, its high cost and the unsatisfactory hysteresis of fibers made with PTMEG, PTMEG continues to be the mainstay of spandex production because, to date, no satisfactory substitute has been found.
One potential substitute for PTMEG which has been evaluated is polyoxypropylene glycol (“PPG”) which, in principle, could be used to prepare spandex fibers. Preparation of spandex fibers from a prepolymer made with a polyol component composed primarily of PPG is attractive from an economic point of view because the cost of PPG is significantly lower than that of PTMEG. In addition, fiber prepared from prepolymers made with PPGs exhibit excellent elongation and retractive or holding power. PPGs are inherently easier to handle than PTMEG because they are non-crystallizable, relatively low viscosity liquids with low pour points. By contrast, PTMEGs are typically solids at 20 to 40° C. depending on the grade.
U.S. Pat. No. 3,180,854, for example, discloses a polyurethane/urea fiber based on a prepolymer made with a 2000 Da molecular weight polyoxypropylene glycol. However, the properties of polyoxypropylene-derived spandex fibers are generally inferior to those of fibers based on PTMEG. Consequently, polyoxypropylene glycols have not been utilized commercially in spandex production. See, e.g., the POLYURETHANE HANDBOOK (Gunther Oertel, Ed., Carl Hanser Verlag Pub., Munich 1985, p. 578) which states: “Polypropylene glycols have so far been used as soft segments only in experimental products since they produce inferior elastanes”. (at page 578)
High molecular weight polyoxypropylene glycols made by conventional processes contain high percentages of terminal unsaturation or monofunctional hydroxyl-containing species (“monol”). The monol is believed by many to act as a chain terminator, limiting the formation of the required high molecular weight polymer during chain extension and yielding products which are generally inferior in comparison to PTMEG-derived elastomers.
The majority of polyoxyalkylene polyether polyols are polymerized in the presence of a pH-basic catalyst. For example, polyoxypropylene diols are prepared by the base catalyzed oxypropylation of a difunctional initiator such as propylene glycol. During base catalyzed oxypropylation, a competing rearrangement of propylene oxide to allyl alcohol continually introduces an unsaturated, monofunctional, oxyalkylatable species into the reactor. The oxyalkylation of this monofunctional species yields allyl-terminated polyoxypropylene monols. The rearrangement is discussed in BLOCK AND GRAFT POLYMERIZATION, Vol. 2, Ceresa, Ed., John Wiley & Sons, pp. 17-21.
Unsaturation is measured in accordance with ASTM D-2849-69 “Testing Urethane Foam Polyol Raw Materials,” and expressed as milliequivalents of unsaturation per gram of polyol (meq/g).
Due to the continual formation of allyl alcohol and its subsequent oxypropylation, the average functionality of the polyol mixture decreases and the molecular weight distribution broadens. Base-catalyzed polyoxyalkylene polyols contain considerable quantities of lower molecular weight, monofunctional species. In polyoxypropylene diols of 4000 Da molecular weight, the content of monofunctional species may lie between 30 and 40 mol percent. In such cases, the average functionality is lowered to approximately 1.6 to 1.7 from the nominal, or theoretical functionality of 2.0. In addition, the polyols have a high polydispersity, Mw/Mn due to the presence of the substantial amount of low molecular weight fractions.
Lowering unsaturation and the attendant large monol fraction in polyoxypropylene polyols has been touted as a means for production of polyurethane elastomers having improved properties. For example, use of polyols having a low content of monofunctional species has been suggested as a method for increasing polymer molecular weight; and increased polymer molecular weight has, in turn, been cited as desirable in producing higher performance polymers.
Reducing unsaturation in polyoxyalkylene polyols by lowering catalyst concentration and decreasing the reaction temperature is not feasible because even though low unsaturation polyols may be prepared, the reaction rate is so slow that oxypropylation takes days or even weeks. Thus, efforts have been made to discover catalysts capable of producing polyoxypropylated products in a reasonable amount of time without introducing monofunctionality due to allylic species.
In the early 1960's, double metal cyanide catalysts such as zinc hexacyano-cobaltate complexes were developed to accomplish this objective. Such complexes are disclosed in U.S. Pat. Nos. 3,427,256; 3,427,334; 3,427,335; 3,829,505; and 3,941,849. Although the unsaturation level is lowered to approximately 0.018 meq/g, the cost of these catalysts coupled with the need for lengthy and expensive catalyst removal steps prevented commercialization of processes for the production polyoxyalkylene polyols using these catalysts.
Other alternatives to basic catalysts such as cesium hydroxide and rubidium hydroxide are disclosed in U.S. Pat. No. 3,393,243. Barium and strontium oxide and hydroxide catalysts (disclosed in U.S. Pat. Nos. 5,010,187 and 5,114,619) enabled modest improvements with respect to unsaturation levels. However, catalyst expense, and in some cases, toxicity, and the modest level of improvement attributable to these catalysts, mitigated against their commercialization. Catalysts such as calcium naphthenate and combinations of calcium naphthenate with tertiary amines have proven to be useful in preparing polyols with unsaturation levels as low as 0.016 meq/g, and more generally in the range of from 0.02 to 0.04 meq/g. (See, e.g., U.S. Pat. Nos. 4,282,387; 4,687,851; and 5,010,117.)
In the 1980's, double metal cyanide complex (DMC) catalysts were revisited. Improvements in catalytic activity and catalyst removal methods encouraged commercial use of DMC catalyzed polyols having low unsaturation levels (in the range of from 0.015 to 0.018 meq/g) commercially for a brief time. However, base catalysis continued to be the primary method used to produce polyoxypropylene polyols. pH-basic catalysts continue to be the catalysts which are primarily used in commercial polyoxyalkylene polyol production processes.
Major advances in DMC catalysts and polyoxyalkylation processes have enabled preparation of ultra-low unsaturation polyoxypropylene polyols on a commercial scale. High molecular weight polyols (molecular weight in the 4000 Da to 8000 Da range) typically exhibit unsaturation levels in the range of from 0.004 to 0.007 meq/g when catalyzed by these improved DMC catalysts. At these levels of unsaturation, only 2 mol percent or less of monofunctional species is present. GPC analysis of these polyols shows them to be virtually monodisperse, often exhibiting polydispersities of less than 1.10. Several such polyols have recently been commercialized as ACCLAIM™ polyols.
U.S. Pat. No. 5,340,902 discloses that unsaturation levels of less than 0.03 milliequivalents/g are beneficial in the production of spandex fibers but does not provide any examples which illustrate use of polyols having unsaturation levels of less than 0.03 meq/g in the production of spandex fibers. U.S. Pat. No. 5,691,441 discloses that blends of low monol polyols with unsaturation levels of less than 0.010 meq/g are needed to achieve the benefits of the disclosed invention.
U.S. Pat. No. 5,691,441 also teaches that “The ultra-low unsaturation polyols have been found to be quantitatively different than both conventional polyols and low unsaturation polyols”. In view of this teaching, it would be expected that the properties of spandex made with blends of PTMEG and polyoxypropylene diols having relatively high levels of unsaturation (greater than 0.010 meq/g) would have properties which are markedly inferior to those of fibers based on blends of PTMEG and polyoxypropylene diols containing ultra-low levels of unsaturation (i.e., less than 0.010 meq/g). U.S. Pat. No. 5,691,441 further teaches that the polyoxypropylene diols containing ultra-low levels of unsaturation should be used in quantities of from 5 to 50 equivalent percent of the total polyol component.
It would, however, be more economically advantageous to be able to use as much of the cheaper PPG and as little of the more expensive PTMEG as possible to produce spandex fibers from polyurethane/ureas.